Some folks have asked about the pwaribbon.xml file and modifying the ribbon in PWA for Project Server 2010. The SDK article Walkthrough: Customizing the PWA Ribbon and Accessing the JS Grid includes that information, although the article can be overlooked if you are not concerned with the JS Grid control. Here is the introductory information in the article about the pwaribbon.xml file:
Modifying the Project Web App Ribbon with an Elements.xml File
The Server ribbon for Project Web App is defined in the pwaribbon.xml file in [Program Files]\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\FEATURES\PWARibbon\listtemplates. To modify the Project Web App ribbon, you can use a custom Elements.xml file to add, change, or remove tabs, groups, and controls in the ribbon. For general information about modifying the Server ribbon in SharePoint applications, see Server Ribbon XML.
You should not modify the pwaribbon.xml file itself. Updates and service packs can replace the pwaribbon.xml file and overwrite your changes. Changes that you make in the pwaribbon.xml file are not supported.
You can also search MSDN for PWA ribbon. The article is at the top of the list.